In the fields of application of a diamond electronic device and an optical device, respectively, it is important to develop an ohmic electrode which has excellent low contact resistance and heat resistance. It has been known that Patent Documents 1 to 5 each describe a technology for forming an ohmic electrode on a semiconductor diamond as a prior art technology. For example, there has been introduced a technology whereby a metal layer is provided on a semiconductor diamond through the intermediary of a graphite layer to thereby form an ohmic junction (Patent Document 1).
Further, as a method for forming an ohmic electrode against a p-type semiconductor diamond, there has been introduced the method whereby elements, such as Ti, Mo, Ta, Zr, Hf, and so forth, reacting with a diamond to form a metal carbide is deposited on the diamond to be followed by a heat treatment applied at not lower than 400° C., thereby forming an ohmic electrode (refer to Patent Documents 2, 3).
Furthermore, as the method for forming an ohmic electrode against an n-type semiconductor diamond, there has been tried the method whereby Ti capable of forming a metal carbide is deposited on the diamond, and a heat treatment is applied thereto at not lower than 600° C. However, with this method, it has been difficult to form an ohmic electrode because of the presence of a barrier formed at an interface (Non-patent Document 1). Meanwhile, use is made of the method whereby Al that is small in work function is formed in a layer added with dopant ions of not less than 1.0×1020 cm−3 (Patent Document 4). However, in this case, a problem remains in that insufficiency in heat resistance, and rectifying properties, is observed. It has been reported that if a metal-diamond transition phase is formed by the implantation of Ga ions, an ohmic electrode can be formed without the application of a heat treatment (Patent Document 4). However, an ion implantation layer cannot be used as an electrode, so that there remain problems in terms of processing, including the necessity of further forming a surface layer of Au, and so forth.
Further, it has been known that for an ohmic electrode on a hydrogen-terminated semiconductor diamond, use is made of gold and platinum, and for a Schottky-barrier forming electrode, use is made of copper, nickel, tungsten, iron, chromium, and so forth (Patent Document 6).
Furthermore, it has been known that a contact resistance was evaluated by the c-TML method using a Ti electrode having an intermediate layer highly doped with phosphorus ions on the order of 1.2×1020 cm−3 on an n-type semiconductor diamond, and the contact resistance was at 2×10−3 Ωcm2 (refer to Non-patent Document 4). Even if the Ti electrode has an intermediate layer that is highly doped, a heat treatment at 420° C. is required of the Ti electrode. Herein, the c-TML method is a method whereby a dual-electrode structure is comprised of a circular electrode, and a doughnut-shaped electrode, combined with each other so as to form a concentric ring structure, a plurality of the dual-electrode structures are prepared, differing in inter-electrode distance from each other, and current/voltage characteristics at the respective inter-electrode distances are measured, thereby working out (hereinafter referred to also as “estimating”) a contact resistance on the basis of the current/voltage characteristics.